In this short piece on the RPA Spotlight, Marshall speculates that the over-structured, over-scheduled life of city kids precludes the sort of open, unstructured playtime that is so important for growth. This is probably true for non-city kids too, but city kids have something non-city kids don’t: vibrant, mixed, monitored sidewalks that are open to anyone. Marshall writes that while "the classic image of kids and city life is of reckless, unescorted children playing in the street," in fact, it is on the street where kids learn to cooperate, innovate, self-govern, and--if the street is the right kind of street--encounter difference (this is the case for Marshall's kids, who live across the street from low-income housing). Thus we take Marshall to be saying that it’s a shame city kids don’t make better use of this resource: over-structured, over-scheduled life is the stuff of the suburbs, where city sidewalks don’t exist, and where encounters have to be planned.
So on the one hand the piece is a variant of the Jacobsian theme of efficiency v. inefficiency, talked about in Economy of Cities in the context of inefficient but innovative Birmingham v. efficient but stagnant Manchester (but also throughout Death and Life of Great American Cities in the context of, well, city kids, parks, and sidewalks). The point here is that if you're not obligated to a prexisting plan, you'll be much more open to making up a new one.
On the other hand, the piece is about the sidewalk, which is perfectly inefficient. It is for this reason that playing on the sidewalk gets a spot in our Arsenal of Inclusion.